The present invention relates to a circuit arrangement and a method for load diagnosis of a semiconductor switch.
In integrated power switches, many applications require a diagnostic function which, by way of example, serves for identifying an interruption of the line connection to a load connected to the semiconductor switch, or a short circuit of the output to which the load is connected, with respect to a supply potential. Such diagnostic functions are employed both for high-side switches, in which the load is located between the semiconductor switch and a reference-earth potential and for low-side switches, in which the load is located between a supply potential, for example a battery, and the semiconductor switch.
In semiconductor switches which can be used both as high-side switches and as low-side switches, a diagnostic circuit that carries out the diagnosis has to be doubly provided, namely a first diagnostic circuit at a first load terminal and a second diagnostic circuit at a second load terminal of the semiconductor switch. Depending on the instantaneous interconnection of the semiconductor switch and the load, either the results of the first or the second diagnostic circuit are used in this case, which necessitates an initial initialization of the diagnostic circuit depending on the interconnection of the semiconductor switch and the load.
In customary diagnostic circuits a constant current is impressed at the output to which the load is connected, and the voltage level established at the output terminal is used for diagnosis. What is disadvantageous in this case is that, owing to the assessment of voltage levels, this concept is susceptible to so-called earth offsets, that is, fluctuations in the reference-earth potential with respect to which a voltage level is to be determined. Moreover, the diagnosis in known circuits in this case relates only to one connecting terminal of the semiconductor switch, for example, the drain terminal in a power MOSFET used as a low-side switch, or the source terminal in a power MOSFET used as a high-side switch. In this case, the known circuits are unable to carry out a diagnosis if the semiconductor switch is connected up in floating fashion, that is, if a load is located between supply potential and a first load terminal and a load is additionally located between the second load terminal and reference-earth potential.